Globopentaose (Gb5), fucosyl-Gb5 (Globo H), and sialyl-Gb5 (SSEA4) are globo-series glycosphingolipid and were first discovered in 1983 in cultured human teratocarcinoma cell line[1] and subsequently found in several malignant cancers.[2],[3] Report showed Globo H overexpression in up to 61%, Gb5 overexpression in 77.5% and SSEA4 overexpression in 95% in breast cancer patients.[4] On the other hand, HER2 gene, the target for therapeutic monoclonal antibody Trastuzumab (Herceptin) that interferes with the HER2/neu receptor, is overexpressed in only 25% breast cancer patients[5]. The comparison clearly demonstrated that the glycosphingolipid antigens (Gb5 and its derivative, Globo H and SSEA4) are better candidates to be developed into cancer vaccine. Hence, Globo H has been conjugated to the keyhole limpet hemocyanin (KLH) as a cancer vaccine, and is under phase III clinical trial in some country now.[6]
There are several disadvantages of current methods used for the synthesis of Gb5, Globo H and SSEA4. The traditional chemical synthesis is tedious and labor-consuming, and several protection and de-protection steps are necessary to achieve high purity and correct stereotype and always lead to the very low total yields. Till now there are many reports for the chemical synthesis of Globo H[7][8][9][10][11][12][13][14] However, only two reports have been published for SSEA4 synthesis. Hsu et al reported a one-pot chemical synthesis approach to assembled the glycan part of SSEA-4 in 24% yield[15] Zhen et al. reported the use of a chemoenzymatic method to synthesize SSEA-4 in milligram scale.[16] On the other hand, the enzymatic synthesis of Globo H based on Leloir-type glycosyltransferase only requires the active nucleotide sugar as donor to catalyze the glycosylation reaction. Nonetheless, the nucleotide sugar is too expensive to synthesize in large scale. Moreover, the by-product pyrophosphate and nucleoside diphosphate inhibit the nucleotide sugar formation of pyrophosphorylase[15] and Leloir-type glycosyltransferase; therefore, how to develop a regeneration strategy is necessary to overcome the limitation and to recharge the nucleotide to achieve constitute nucleotide sugar product in order to continue the reaction. During the past several years, many groups worked to tackle the major problem of nucleotide sugar regeneration and most of the sugar nucleotide regeneration have been solved. However, there is still some space to improve the technology of sugar nucleotide regeneration, especially the UDP-Gal regenerate is much difficult. For example, UDP-Gal regeneration was first proposed in 1982 by Wong and Whiteside via UDP-Glc C4 epimerase to interconverse UDP-Glc and UDP-Gal ([17]). Ten years later, our group developed the secondary UDP-Gal regeneration method. Instead of using UDP-Glc C4 epimerase, Glc-1-phosphate uridylyltransferase located in galactose operon in E. coli was used to interchange Gal-1-phosphate and UDP-Glc to Glc-1-phosphate and UDP-Gal.[18] However, the final pathway to directly condense UTP and Gal-1-phosphate to form UDP-Gal was not established due to the absence of suitable enzyme. Because the target compounds Gb5, Globo H and SSEA4 are Gal-related molecules, how to overcome the major difficult of UDP-Gal regeneration and increase its efficiency will be the key point for large scale enzymatic synthesis of Gb5, Globo H and SSEA4.
In summary, there are several limitations to current methods of large scale synthesizing Gb5, Globo H and SSEA4 in the art. Thus, there is a need for new synthetic procedures that produce Gb5, Globo H, SSEA4, and intermediates thereto in an efficient manner.